The invention is directed generally to television receivers and, more particularly, to a pulse width limiter for extracting horizontal sync information from a composite sync signal which includes both horizontal and vertical sync pulses.
Composite video signals received by a television receiver include video components as well as horizontal and vertical synchronizing (sync) pulses. In a typical television receiver, a sync separator receives the composite video signal, strips the video components therefrom, and outputs a composite sync signal containing horizontal and vertical sync pulses. The composite sync signal developed by the sync separator is used to synchronize the receiver's horizontal and vertical scan with the composite video signal.
To lock the receiver's horizontal scan to the incoming horizontal sync pulses, the latter pulses are typically applied to a phase detector which also receives horizontal rate flyback pulses developed by the receiver. The phase detector generates an error signal indicative of any frequency and/or phase difference between its inputs, and the error signal is used to adjust the operation of the receiver's horizontal oscillator until phase lock is achieved.
Because such phase detectors are sensitive to the widths of the sync pulses they receive, it is desirable to limit the widths of the applied sync pulses to a maximum value. This is particularly true of the broad sync pulses typically broadcast during the field sync interval which occurs during vertical retrace. The application of those broader sync pulses to the horizontal phase detector may generate a faulty error signal if the horizontal phase detector has some nonlinearity and/or offset. The faulty error signal then shifts the frequency of the horizontal oscillator to an incorrect value. After the broad pulses are completed, the phase detector, upon receiving normal width sync pulses, brings the frequency of the horizontal oscillator back to its correct value. However, depending on the phase detector's time constant, a top hook may be visible on the screen in vertically shaped objects.
Another reason for limiting the pulse width of the horizontal sync pulses is because of the receiver's operation during channel changing by a user. During the intermittent period between channels, there is no video signal. When the signal reappears, the RF (radio-frequency) amplifier and the IF (intermediate-frequency) amplifier are at maximum gain. This condition may cause an overload wherein the composite video input to the sync separator appears as a very broad sync pulse. Consequently, the horizontal oscillator is pulled toward an incorrect frequency in a manner similar to the situation which occurs when the field sync pulses are received. Since automatic gain control of the RF and IF amplifiers is dependent on the horizontal oscillator being phase locked with the incoming sync pulses, a longer period of time is required for the RF and IF amplifiers to reduce gain.
Due to the aforementioned problems, prior sync separators have employed a differentiating process wherein the composite sync signal is passed through a high pass filter to limit the width of the horizontal sync pulses. The high pass filter is typically formed by an external or on-chip RC (resistor-capacitor) network. However, the space requirements and the reliability problems inherent in an RC filter network are undesirable.
The present invention overcomes the problems cited above by providing an improved differentiation process wherein the pulse widths of horizontal sync pulses are restricted by a digital pulse width limiter.